FRIDAY, Dec. 8, 2017 (HealthDay News) -- If your pooch responds well to your smiling face, the "love hormone" oxytocin might have something to do with it, new research suggests.

Produced naturally by humans and dogs, the hormone "influences what the dog sees and how it experiences the thing it sees," said study co-author Sanni Somppi. She's a graduate student at the University of Helsinki, Finland.

Somppi worked with lead investigator Outi Vainio as part of the university's Canine Mind research group.

They tested the impact of giving extra doses of oxytocin -- long known to influence bonding -- on 43 dogs. The testing was done in two stages.

First, the dogs were given oxytocin and observed as they looked at either smiling human faces or angry human faces on a computer monitor.

Dogs can't tell us their emotional states, of course, so the researchers relied on "gazing behavior," as well as pupil dilation, which the researchers said is a good indicator of emotional responses.

"We were among the first researchers in the world to use pupil measurements in the evaluation of dogs' emotional states. This method had previously only been used on humans and apes," Vainio said in a university news release.

According to the study, without the added dose of oxytocin, the dogs' pupils were larger when staring at an angry face. That wasn't a surprise, since dogs usually react most strongly to stimulation that's threatening or frightening, Vanio's team said.

But the situation was different when pooches got the extra oxytocin.

In that scenario, the dogs' pupils grew bigger when facing a smiling face -- indicating a greater interest in the friendlier image than in the angrier one.

The study can't prove that oxytocin helps canines connect with a happy human, Vanio's team said. But it does suggest that the hormone softens the perceived threat of angry faces, while simultaneously making smiling faces more appealing.

"Both effects promote dog-human communication and the development of affectionate relations," Vainio said.

The findings were published recently in the journal Frontiers in Psychology.